Motor driving apparatus

CROSS REFERENCE TO RELATED APPLICATIONS

This is a U.S. national stage of application No. PCT/JP2005/023322, filed on 20 Dec. 2005. Priority under 35 U.S.C. §119(a) and 35 U.S.C. §365(b) is claimed from Japanese Application No. 2004-371335, filed 22 Dec. 2004,the disclosure of which is also incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a motor driving apparatus and it particularly relates to a technique for controlling the rotation frequency of a motor.

2. Description of the Related Art

In a motor driving apparatus that rotates a DC motor or spindle motor at a desired rotation frequency thereof, the rotation frequency of a motor is monitored and a driving signal is generated based on a deviation thereof from a desired target value.

There are cases where a speed discriminator as described in Patent Document 1, for example, is used in the motor driving apparatus that controls the motor in this manner.

The speed discriminator compares the current rotation frequency of the motor with the rotation frequency that serves as the desired target value, and outputs an acceleration pulse or a deceleration pulse according to the deviation. This acceleration pulse and the deceleration pulse are converted to the DC voltage, so that a control voltage for driving the motor is produced.

[Patent Document 1]

Japanese Patent Application Laid-Open No. Hei06-30589.

As described above, the speed discriminator generates the acceleration and deceleration pulses based on the deviation of the current rotation frequency of the motor from the target value of the rotation frequency thereof, so that a feedback path is formed in the motor driving apparatus. A low-pass filter for smoothing the control voltage obtained by converting the acceleration and deceleration pulses is provided in this feedback path.

In such a motor driving apparatus, when the rotation of the motor is started from a motor stoppage state, an acceleration pulse is generated from the speed discriminator and thus the control voltage for driving the motor rises up rapidly. Nevertheless, since high-frequency components are removed by the low-pass filter, a starting time required until the rotation frequency of the motor reaches a desired target value will become longer due to the effect of a time constant of the low-pass filter. Also, since the bandwidth of a feedback path is narrowed by the low-pass filter, an overshoot that exceeds considerably the desired target value may occur and a ringing may occur.

SUMMARY OF THE INVENTION

The present invention has been made in view of the foregoing problems, and a general purpose thereof is to provide a motor driving apparatus which shortens the starting time of a motor.

One embodiment of the present invention relates to a motor driving apparatus. This motor driving apparatus comprises: a control unit which detects a rotation frequency of a motor to be driven and generates a control voltage in a manner such that the rotation frequency thereof is brought close to a desired rotation frequency; a drive unit which drives the motor based on the control voltage generated by the control unit; and a start circuit which fixes the control voltage to a predetermined initial voltage at the start of driving the motor.

According to this embodiment, the control voltage is set to a predetermined initial voltage by the start circuit. Thereby, that the control voltage rises up precipitously and the motor starts rotating at full torque is prevented, so that the starting time required until a desired rotation frequency is reached can be reduced.

The start circuit may adjust the initial voltage according to a drive condition of the motor.

The “drive condition of the motor” includes various conditions such as a target value of the motor, the type of the motor, the ambient temperature and the supply voltage. There are cases where a difference in the drive condition of the motor results in a change of an optimum initial voltage. Thus, the starting time can be further preferably shortened by so configuring the start circuit as to be adjustable according to each condition.

The initial voltage may be stored in a register according to each of the various drive conditions. Then the optimum start-up control can be performed by reading out an initial value from the register, as appropriate, according to the drive condition.

The control circuit further includes a clamping circuit which sets an upper limit and a lower limit of the control voltage, and the start circuit may be set active for a predetermined period of time at the start of driving the motor, and the clamping circuit may be set active after the predetermined period of time has elapsed.

Since the motor driving apparatus sets an upper limit and a lower limit of motor torque, there are cases where it is equipped with a clamping circuit that clamps the control voltage. In this case, the control unit sets the start circuit active at the start of driving a motor and, thereafter, sets the clamping circuit active so as to perform a normal operation. As a result the staring time of the motor can be shortened.

The clamping circuit maybe structured integrally with the start circuit, and at least one of the upper limit and the lower limit of the control voltage may be set to the initial voltage for a predetermined period of time at the start of driving the motor.

Also, the control voltage can be fixed to the initial voltage by setting the clamping voltage of the clamping circuit to the initial voltage. The circuitry can be simplified.

The control unit may include: a speed discriminator which generates an acceleration pulse and a deceleration pulse based on a deviation of a speed signal indicating the rotation frequency of the motor from the desired rotation frequency; and a voltage generator which generates the control voltage based on a pulse generated by the speed discriminator. The start circuit may fix the speed signal inputted to the speed discriminator at the start of driving the motor, to a predetermined initial value.

Before the rotation frequency of a motor rises immediately after the start of the driving of the motor, the speed signal inputted to the speed discriminator takes a low value but this speed signal is fixed to a predetermined initial value. Thus, the control unit generates a control voltage fixed to a certain value and thereby the starting time can be shortened.

The start circuit may adjust a period of time during which the control voltage is fixed to the initial voltage, according to a drive condition of the motor.

By varying the period, during which the start circuit is being set active, according to a drive condition of the motor, the starting time can be further preferably shortened.

The start circuit may fix the control voltage for a period of time from the start of driving the motor until the rotation frequency of the motor reaches a predetermined rotation frequency. The predetermined rotation frequency may be set based on a target value of the rotation frequency of the motor.

It is to be noted that any arbitrary combination or rearrangement of the above-described structural components and so forth is effective as and encompassed by the present embodiments.

Moreover, this summary of the invention does not necessarily describe all necessary features so that the invention may also be a sub-combination of these described features.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which:

<figref idref="DRAWINGS">FIG. 1</figref> shows a structure of a motor driving apparatus according to an embodiment;

<figref idref="DRAWINGS">FIG. 2</figref> is a signal waveform diagram showing an operating state of a motor driving apparatus when a start circuit is not activated;

<figref idref="DRAWINGS">FIG. 3</figref> is a signal waveform diagram showing an operating state of a motor driving apparatus when a start circuit is operated;

<figref idref="DRAWINGS">FIG. 4</figref> is a circuit diagram showing a modification of a motor driving apparatus of <figref idref="DRAWINGS">FIG. 1</figref>;

<figref idref="DRAWINGS">FIG. 5</figref> is a circuit diagram showing another modification of a motor driving apparatus of <figref idref="DRAWINGS">FIG. 1</figref>; and

<figref idref="DRAWINGS">FIG. 6</figref> is a circuit diagram showing still another modification of a motor driving apparatus of <figref idref="DRAWINGS">FIG. 1</figref>.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described based on preferred embodiments which do not intend to limit the scope of the present invention but exemplify the invention. All of the features and the combinations thereof described in the embodiment are not necessarily essential to the invention.

<figref idref="DRAWINGS">FIG. 1</figref> shows a structure of a motor driving apparatus <b>100</b> according to an embodiment. A motor <b>200</b> to be driven is also shown in <figref idref="DRAWINGS">FIG. 1</figref> together with the motor driving apparatus <b>100</b>.

This motor driving apparatus <b>100</b> monitors the rotation frequency of the motor <b>200</b>, and drives the motor <b>200</b> so that the rotation frequency thereof is brought close to a desired target value. The rotation frequency of the motor <b>200</b> is detected by an unshown speed detection unit, and the speed information thereof is inputted to a speed terminal <b>102</b> as a speed signal SPD. For instance, the speed detection unit may convert a current flowing to the motor into speed information by detecting the current or may detect the speed using a Hall element. In the present embodiment, it is assumed that the speed signal SPD is a periodic signal having a frequency proportional to the rotation frequency of a motor.

The motor driving apparatus <b>100</b> includes a control unit <b>110</b>, a drive unit <b>120</b>, and a start circuit <b>40</b>.

The control unit <b>110</b> compares the speed signal SPD indicating the current rotation frequency of a motor inputted to the speed terminal <b>102</b> against the target value of the rotation frequency, and generates a control voltage Vcnt based on the deviation therefrom. The motor driving apparatus <b>100</b> includes an enable terminal <b>104</b>, and it drives the motor <b>200</b> when an enable signal EN is at a high level.

This control unit <b>110</b> includes a control voltage generator <b>10</b>, a clamping circuit <b>20</b>, and a filter <b>30</b>.

The control voltage generator <b>10</b> includes a speed discriminator <b>12</b>, a first constant current source <b>14</b>, a second constant current source <b>16</b>, a first switch SW<b>1</b>, and a second switch SW<b>2</b>.

The speed discriminator <b>12</b> compares the frequency of a reference clock signal REF indicating target value of the rotation frequency of the motor <b>200</b> against the speed signal SPD. When the rotation frequency of the motor <b>200</b> is lower than the target value, the speed discriminator <b>12</b> outputs an acceleration pulse SIG<b>1</b> for a time duration corresponding to the difference therebetween. When the rotation frequency of the motor <b>200</b> is higher than the target value, the speed discriminator outputs a deceleration pulse SIG<b>2</b> for a time duration corresponding to the difference therebetween.

The first constant current source <b>14</b>, the second constant current source <b>16</b>, the first switch SW<b>1</b>, the second switch SW<b>2</b> and the filter <b>30</b> constitutes a control voltage generator which converts the acceleration pulse SIG<b>1</b> and the deceleration pulse SIG<b>2</b> generated by the speed discriminator <b>12</b> into a DC control voltage Vcnt.

The first constant current source <b>14</b> is a current supply for charging a first capacitor C<b>30</b>, whereas the second constant current source <b>16</b> is a current supply for discharging the first capacitor C<b>30</b>. The first constant current source <b>14</b> is connected with the first capacitor C<b>30</b> via the first switch SW<b>1</b>. Similarly, the second constant current source <b>16</b> is connected with the first capacitor C<b>30</b> via the second switch SW<b>2</b>.

Now, when the acceleration pulse SIG<b>1</b> is outputted, the first switch SW<b>1</b> turns on and the first capacitor C<b>30</b> is charged by the first constant current source <b>14</b>, so that the control voltage Vcnt rises. Conversely, when the deceleration pulse SIG<b>2</b> is outputted, the second switch SW<b>2</b> turns on and the first capacitor C<b>30</b> is discharged by the second constant current source <b>16</b>, so that the control voltage Vcnt drops.

The filter <b>30</b> includes a first capacitor C<b>30</b>, a second capacitor C<b>32</b>, and a resistor R<b>30</b>. Since the control voltage Vcnt is generated by repeating the charging and the discharging by the acceleration pulse SIG<b>1</b> and the deceleration pulse SIG<b>2</b>, it contains a high-frequency noise component. The high-frequency component in the control voltage Vcnt is removed by a low-pass filter effect of this filter <b>30</b>.

In this manner, the control voltage Vcnt is outputted from the control unit <b>110</b> so that the rotation frequency of the motor <b>200</b> to be driven is brought close to a desired target value.

The clamping circuit <b>20</b> sets an upper limit Vcu and a lower limit Vcl of the control voltage Vcnt. The clamping circuit <b>20</b> includes a first transistor Q<b>1</b>, a second transistor Q<b>2</b>, a third switch SW<b>3</b>, and a fourth switch SW<b>4</b>.

The first transistor Q<b>1</b> is an NPN-type bipolar transistor where a minimum clamping voltage Vc<b>1</b> is applied to a base terminal. An emitter terminal is connected to an output terminal of the control unit <b>110</b> via the third switch SW<b>3</b>.

When the control voltage Vcnt drops with the third switch SW<b>3</b> being on, the first transistor Q<b>1</b> turns on. With the first transistor Q<b>1</b> being on, the voltage Vel of the emitter terminal is clamped to Vcl=Vc<b>1</b>−Vbe, by the minimum clamping voltage Vcl applied to the base terminal, where the voltage Vbe denotes a forward voltage between the base and the emitter of the bipolar transistor.

Similarly, the second transistor Q<b>2</b> is a PNP-type bipolar transistor where a maximum clamping voltage Vc<b>2</b> is applied to a base terminal. And an emitter terminal is connected to the output terminal of the control unit <b>110</b> via the fourth switch SW<b>4</b>.

When the control voltage Vcnt rises with the fourth switch SW<b>4</b> being on, the second transistor Q<b>2</b> turns on. Then the voltage Vcnt, which is an emitter voltage thereof, is clamped to Vcu=Vc<b>2</b>+Vbe.

The on and off of the third switch SW<b>3</b> and the fourth switch SW<b>4</b> is controlled from outside according to an operating state of the motor driving apparatus <b>100</b>.

At the start of driving the motor <b>200</b>, the start circuit <b>40</b> fixes the control voltage Vcnt to a predetermined initial voltage Vinit. This start circuit <b>40</b> includes a fifth switch SW<b>5</b> and a buffer <b>42</b>. The initial voltage Vinit is inputted to the buffer <b>42</b>, and the output thereof is connected to the output terminal of the control unit <b>110</b> via the switch SW<b>5</b>. As the fifth switch SW<b>5</b> turns on, the control voltage Vcnt appearing at the output terminal of the control unit <b>110</b> is fixed to the initial voltage Vinit. Similar to the above-described third switch SW<b>3</b> and the fourth switch SW<b>4</b>, the on and off of the fifth switch SW<b>5</b> is also controlled from outside according to an operating state of the motor driving apparatus <b>100</b>.

The control voltage Vcnt is inputted to the drive unit <b>120</b>. The drive unit <b>120</b>, which includes a pulse width modulator <b>50</b> and an output unit <b>60</b>, drives the motor <b>200</b> based on the control voltage Vcnt generated by the control unit <b>110</b>.

The pulse width modulator <b>50</b>, which includes a voltage comparator <b>52</b> and an oscillator <b>54</b>, generates a pulse-width-modulation (PWM) signal Vpwm which varies the on period based on the inputted control voltage Vcnt.

The oscillator <b>54</b> outputs a periodic voltage Vosc of triangular wave or sawtooth wave pattern.

The control voltage Vcnt and the periodic voltage Vosc are inputted to the voltage comparator <b>52</b> from the control unit <b>110</b> and the oscillator <b>54</b>, respectively. The voltage comparator <b>52</b> compares the control voltage Vcnt against the periodic voltage Vosc, and it outputs a high level when Vosc&gt;Vx and it outputs a low level when Vosc&lt;Vx. The PWM signal Vpwm outputted from the voltage comparator <b>52</b> is a pulse-width-modulated signal in which the period of high level and low level varies according to the level of the control voltage Vcnt.

Here, the control voltage Vcnt outputted from the control unit <b>110</b> is a voltage that varies according to the deviation of the rotation frequency of the motor <b>200</b> from a target value thereof. Thus, the duty ratio of the PWM signal Vpwm varies in a manner such that the rotation frequency of the motor <b>200</b> is brought close to the target value. The PWM signal Vpwm generated by the pulse width modulator <b>50</b> is inputted to the output unit <b>60</b>.

The output unit <b>60</b> includes a pre-driver circuit <b>62</b>, switching transistors M<b>1</b> to M<b>4</b> and a detection resistor Rd, and the motor <b>200</b> is driven based on the PWM signal Vpwm.

The switching transistors M<b>1</b> to M<b>4</b> are each a MOSFET (Metal Oxide Semiconductor Field Effect Transistor), performs a switching operation according to a voltage applied to a gate terminal thereof, and supplies a drive voltage intermittently to the motor <b>200</b>. These switching transistors M<b>1</b> to M<b>4</b> constitute an H-bridge circuit. With the switching transistors M<b>1</b> and M<b>3</b> being off, turning on and off the switching transistors M<b>1</b> and M<b>4</b> synchronously applies a supply voltage Vdd to one terminal of the motor <b>200</b> and applies a voltage close to ground voltage to the other terminal so as to be able to rotate the motor <b>200</b> in a certain direction. The detection resistor Rd converts a motor current flowing to the motor <b>200</b> into a voltage. The speed signal SPD inputted to the above-described control voltage generator <b>10</b> may be generated from a voltage appearing across the detection resistor Rd.

The pre-driver circuit <b>62</b> controls the on and off of the switching transistors M<b>1</b> to M<b>4</b>, based on the PWM signal Vpwm outputted from the pulse width modulator <b>50</b>. During an ON period of the PWM signal Vpwm, the pre-driver circuit <b>62</b> turns on either a pair of switching transistors M<b>1</b> and M<b>4</b> or a pair of switching transistors M<b>2</b> and M<b>3</b> so as to apply the drive voltage to the motor <b>200</b>. Thus, the longer the ON period of the PWM signal Vpwm, the drive voltage is applied to the motor <b>200</b> with the result that the motor <b>200</b> is rotated with a larger torque, namely, a higher rotation frequency.

An enable signal EN is inputted to the pulse width modulator <b>50</b> and the output unit <b>60</b>. When the enable signal EN goes to a high level, the motor <b>200</b> is driven; and when it goes to a low level, the driving of the motor <b>200</b> is stopped. When the enable signal EN is at a low level, the pre-driver circuit <b>62</b> and/or the voltage comparator <b>52</b> may be turned off for power saving purposes.

A description will now be given of an operation of the motor driving apparatus <b>100</b> structured as above.

To further clarify the effects of the motor driving apparatus <b>100</b>, a description is first given of a case where the start circuit <b>40</b> is not activated but the clamping circuit <b>20</b> only is set active. <figref idref="DRAWINGS">FIG. 2</figref> is a signal waveform diagram showing an operating state of the motor driving apparatus <b>100</b> when the start circuit <b>40</b> is not operated.

The rotation of the motor <b>200</b> is stopped before time T<b>0</b> and therefore the speed signal SPD indicating the rotation frequency of the motor takes a minimum value. At this time, the acceleration pulse SIG<b>1</b> continues to be outputted from the speed discriminator <b>12</b>, so that the control voltage Vcnt rises and is clamped to the upper limit Vcu by the clamping circuit <b>20</b>.

At time T<b>0</b>, the enable signal EN goes to a high level and the start of the driving of the motor <b>200</b> is specified. As the enable signal EN goes to the high level, the drive unit <b>120</b> starts driving the motor <b>200</b>, at full torque, based on the control voltage Vcnt. At the start of the rotation, the rotation frequency starts rising with a delay from time T<b>0</b> due to a static friction force exerted on the motor <b>200</b>. Then, the rotation frequency of the motor <b>200</b> rises up precipitously and exceeds a target value REF.

As the rotation frequency of the motor <b>200</b> exceeds the desired target value REF, the deceleration pulse SIG<b>2</b> is outputted from the speed discriminator <b>12</b> and the control voltage Vcnt drops. A the same time, since high-frequency components are removed by the filter <b>30</b> provided in a feedback loop, the response cannot follow the rapid increase in rotation frequency of the motor <b>200</b> and therefore the rotation frequency of the motor <b>200</b> results in a large overshoot. Thereafter, the control voltage Vcnt gradually approaches the target value REF.

In this manner, if the start circuit <b>40</b> is not activated, problems will arise where a longer starting time is required and the rotation frequency suffers an overshoot.

Next, a description will be given of a case where the start circuit <b>40</b> is operated. <figref idref="DRAWINGS">FIG. 3</figref> is a signal waveform diagram showing an operating state of the motor driving apparatus <b>100</b> when the start circuit <b>40</b> is operated.

Before time T<b>0</b>, the fifth switch SW<b>5</b> turns on and the start circuit <b>40</b> is set active. At this time, the control voltage Vcnt is fixed to an initial voltage Vinit. This initial voltage Vinit is set to a level lower than the clamping voltage Vcu of <figref idref="DRAWINGS">FIG. 2</figref> and is also set to a level lower than the voltage Vref corresponding to a control target value.

As the enable signal EN goes to a high level, the fifth switch SW<b>5</b> turns off in synchronism with the enable signal EN, and the third switch SW<b>3</b> and the fourth switch SW<b>4</b> turn on. As a result, the clamping circuit <b>20</b> becomes active and the fixation of the control voltage Vcnt is terminated.

The enable signal EN goes to the high level and the drive unit <b>120</b> starts driving the motor <b>200</b>. Since at the start of driving it the control voltage Vcnt is fixed to the initial voltage Vinit which is set lower than the full torque, the motor <b>22</b> increases the rotation frequency more slowly than in the case of <figref idref="DRAWINGS">FIG. 2</figref>. Since the rotation frequency of the motor <b>200</b> rises more slowly than in the case of <figref idref="DRAWINGS">FIG. 2</figref>, a control can be performed so that control signal Vcnt follows the rotation frequency of the motor <b>200</b>. Thus, it can be stabilized to a desired rotation frequency REF in a short length of time without causing an overshoot.

As described above, by employing the motor driving apparatus <b>100</b> according to the present embodiment, at the start of driving the motor <b>200</b> the control voltage Vcnt is fixed to a predetermined initial voltage Vinit by the start circuit <b>40</b>. As a result, the starting time can be shortened and the occurrence of overshoot can be reduced.

In <figref idref="DRAWINGS">FIG. 3</figref>, the switching is made from the start circuit <b>40</b> to the clamping circuit <b>20</b> simultaneously with the instant the enable signal from outside goes to a high level. However, the speed signal SPD may be monitored, and when a predetermined value has been reached, the fifth switch SW<b>5</b> of the start circuit <b>40</b> may be turned off and the third switch SW<b>3</b> and the fourth switch SW<b>4</b> of the clamping circuit <b>20</b> may be turned on. Also, after a predetermined passage of time has elapsed after the enable signal EN became a high level, the switching may be made from the start circuit <b>40</b> to the clamping circuit <b>20</b>.

The start circuit <b>40</b> may adjust the initial voltage Vinit according toa drive condition of the motor <b>200</b>. For instance, a register in which the initial voltage Vinit and the drive condition are brought into correspondence with each other may be prepared in the motor driving apparatus <b>100</b>, and the initial voltage Vinit may be read out from the register, according to the drive condition.

<figref idref="DRAWINGS">FIG. 4</figref> is a circuit diagram showing a modification of the motor driving apparatus <b>100</b> of <figref idref="DRAWINGS">FIG. 1</figref>. In the Figures from here on, the same reference numerals will be given to the same components as those of FIG. l and the repeated explanation thereof is omitted.

A control voltage generator <b>10</b> of <figref idref="DRAWINGS">FIG. 4</figref> includes a first constant voltage source <b>80</b> and a second constant voltage source <b>82</b> in place of the first constant current source <b>14</b> and the second constant current source <b>16</b> of <figref idref="DRAWINGS">FIG. 4</figref>.

As an acceleration pulse SIG<b>1</b> is outputted from a speed discriminator <b>12</b>, a first switch SW<b>1</b> turns on and then the control voltage generator <b>10</b> outputs a first voltage V<b>1</b> outputted from the first constant voltage source <b>80</b>. As a deceleration pulse SIG<b>2</b> is outputted, the second switch SW<b>2</b> turns on and then the control voltage generator <b>10</b> outputs a second voltage V<b>2</b> outputted from the second constant voltage source <b>82</b>.

A filter <b>70</b> includes an operational amplifier <b>72</b>, a voltage source <b>74</b>, resistors R<b>70</b> and R<b>72</b>, and capacitors C<b>70</b> and C<b>72</b> so as to constitute an active filter. A constant voltage V<b>3</b> outputted from the voltage source <b>74</b> is applied to a noninverting input terminal of the operational amplifier <b>72</b>. Either one of the first voltage V<b>1</b> and the second voltage V<b>2</b> outputted from the control voltage generator <b>10</b> is applied to an inverting input terminal of the operational amplifier <b>72</b> via the resistor R<b>70</b>.

The capacitor C<b>70</b> is connected between an output terminal of the operational amplifier <b>72</b> and the inverting input terminal. The capacitor C<b>72</b> and the resistor R<b>70</b> are connected in parallel with the capacitor C<b>70</b>.

The filter <b>70</b> integrates the first voltage V<b>1</b> or the second voltage V<b>2</b> outputted from the control voltage generator <b>10</b>. And high-frequency components of the control voltage Vcnt is removed by the low-pass filter effect of the filter <b>70</b>.

By employing the motor driving apparatus <b>100</b> according to the modification shown in <figref idref="DRAWINGS">FIG. 4</figref>, the same effects as in the motor driving apparatus <b>100</b> shown in <figref idref="DRAWINGS">FIG. 1</figref> can be attained. Further, according to the modification of <figref idref="DRAWINGS">FIG. 4</figref>, the motor drive with higher accuracy can be carried out.

<figref idref="DRAWINGS">FIG. 5</figref> is a circuit diagram showing another modification of the motor driving apparatus <b>100</b> of <figref idref="DRAWINGS">FIG. 1</figref>. In this motor driving apparatus <b>100</b>, a start circuit <b>40</b>′ is structured integrally with a clamping circuit <b>20</b>.

By switching voltages Vc<b>1</b> and Vc<b>2</b> applied to base terminals of a first transistor Q<b>1</b> and a second transistor Q<b>2</b>, respectively, the start circuit <b>40</b>′ of <figref idref="DRAWINGS">FIG. 5</figref> can be operated as a function of the clamping circuit <b>20</b> and the start circuit <b>40</b> of <figref idref="DRAWINGS">FIG. 1</figref>.

Now, when voltages defining a maximum and a minimum torque of a motor <b>200</b> are applied to the base terminals of the first transistor Q<b>1</b> and the second transistor Q<b>2</b>, respectively, it operates as a clamping circuit. When Vc<b>1</b>=Vinit+Vbe and Vc<b>2</b>=Vinit−Vbe are applied to the base terminals of the first transistor Q<b>1</b> and the second transistor Q<b>2</b>, respectively, it operates as a start circuit where a control voltage Vcnt is fixed to the initial voltage Vinit.

An operation of the motor driving apparatus <b>100</b> structured as shown in <figref idref="DRAWINGS">FIG. 5</figref> is described with reference to FIG. <b>3</b> again. Before time T<b>0</b> of <figref idref="DRAWINGS">FIG. 3</figref>, the voltage Vc<b>1</b>=Vinit+Vbe is applied to the base terminal of the first transistor Q<b>1</b>, whereas the voltage Vc<b>2</b>=Vinit−Vbe is applied to the base of the second transistor Q<b>2</b>. This fixes the control voltage Vcnt to the initial voltage Vinit.

When at time T<b>0</b> the base voltages of the first transistor Q<b>1</b> and the second transistor Q<b>2</b> are switched to the voltages defining the maximum and the minimum torque, the fixation of the control voltage Vcnt is terminated. Then the control voltage Vcnt starts rising and the rotation frequency of the motor <b>200</b> approaches a desired rotation frequency.

Similar to the motor driving apparatus <b>100</b> shown in <figref idref="DRAWINGS">FIG. 1</figref>, the starting time can be shortened and the occurrence of overshoot can be reduced by the motor driving apparatus <b>100</b> shown in <figref idref="DRAWINGS">FIG. 5</figref>.

<figref idref="DRAWINGS">FIG. 6</figref> is a circuit diagram showing still another modification of the motor driving apparatus <b>100</b> of <figref idref="DRAWINGS">FIG. 1</figref>. A start circuit <b>40</b> according to this modification fixes a speed signal SPD inputted to a speed discriminator <b>12</b> at the start of driving a motor <b>200</b>, to a predetermined initial value INIT.

The start circuit <b>40</b> shown in <figref idref="DRAWINGS">FIG. 6</figref> includes a periodic signal generator <b>90</b> and a fifth switch SW<b>5</b>. The periodic signal generator <b>90</b> generates an initial signal INIT having a predetermined frequency.

When, at the start of driving the motor, the fifth switch SW<b>5</b> turns on and the start circuit <b>40</b> is set active, this initial signal INIT is inputted to a control voltage generator <b>10</b>. As a result, the speed discriminator <b>12</b> compares the frequency of the initial signal INIT with that of a reference clock signal REF and then outputs an acceleration pulse SIG<b>1</b> and a deceleration pulse SIG<b>2</b>.

According to the start circuit <b>40</b> shown in <figref idref="DRAWINGS">FIG. 6</figref>, the control voltage Vcnt is indirectly fixed by fixing the speed signal SPD. Thus, similar to the motor driving apparatus <b>100</b> shown in <figref idref="DRAWINGS">FIG. 1</figref>, the starting time can be shortened and the occurrence of overshoot can be reduced.

The above-described embodiments are merely exemplary, and it is understood by those skilled in the art that various further modifications to the combination of each component and process thereof are possible and that such modifications are also within the scope of the present invention.

In the embodiments, a description has been given of a case where the control voltage Vcnt is generated by using the speed discriminator <b>12</b> but it is not limited thereto. For example, a voltage appearing at the detection resistor Rd of the output unit <b>60</b> may be used as the speed signal. And an error voltage obtained by amplifying an error from a predetermined reference voltage may be used as a control signal Vcnt and this control Vcnt may be fixed at the start of driving the motor. Also, the control voltage Vcnt maybe fixed indirectly by fixing the voltage appearing at the detection resistor Rd.

In the embodiments, the motor <b>200</b> is driven by an H-bridge circuit but this should not be considered as limiting and the present invention is applicable to other drive methods. In the embodiments, the control voltage Vcnt is pulse-width modulated by the pulse width modulator <b>50</b> so as to drive the motor <b>200</b> based on the PWM signal Vpwm. However, it maybe linear driven based on the control voltage Vcnt.

In the present embodiments, the elements constituting a motor driving apparatus <b>100</b> may all be integrated in a single package, or may be formed by other separate integrated circuits and some of them may be comprised of discrete parts. Which of the parts is to be integrated may be determined in consideration of the cost, the area to be occupied, the usage and the like.

While the preferred embodiments of the present invention have been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the appended claims.